Method of impregnating porous substrates with treating liquids

ABSTRACT

Method for impregnating a porous substrate such as wood with a treating liquid wherein the substrate is immersed in a body of the treating liquid confined in a pressure vessel, the vessel is pressurized and the contents of the vessel are subjected to repetitive pressure pulses over a broad range of frequencies and of varying amplitude to increase the rate of impregnation of treating liquid into substrate.

United States Patent Jackson 11. Barnett, Jr.

84 North Crest, Chattanooga, Tenn. 37404 877,403

Nov. 17, 1969 Jan. 4, 1972 Inventor Appl. No. Filed Patented METHOD OF IMPREGNATING POROUS SUBSTRATES WITH TREATING LIQUIDS 6 Claims, 6 Drawing Figs.

US. Cl 117/113, 21/7,21/65, 117/116, 118/50 Int. Cl 827k 3/08, B44d l/06, 844d 1/26 Field ofSearch 117/113,

116, 59, DIG. 8; 21/7, 63, 65; 118/50, 50.1, 400

Primary Examiner-Alfred l... Leavitt Assistant Examiner-Thomas E. Bokan Attorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT: Method for impregnating a porous substrate such as wood with a treating liquid wherein the substrate is immersed in a body of the treating liquid confined in a pressure vessel, the vessel is pressurized and the contents of the vessel are subjected to repetitive pressure pulses over a broad range of frequencies and of varying amplitude to increase the rate of impregnation of treating liquid into substrate.

PATENTED JAN 4:972

SHEET 1 OF 2 TTORNE Y-S PATENTED JAN 4 I972 SHEET 2 [1F 2 I N VENTOK METHOD OF IMPREGNATING POROUS SUBSTRATES WITH TREATING LIQUIDS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of impregnating wood or a similar porous substrate with treating liquids such as preservatives, utilizing relatively sharply peaked pressure pulses to alternately bring the pressure exerted by the treating liquid above and below the ambient pressure to which it is subjected in the pressurized vessel.

2. Description of the Prior Art This invention is particularly directed to the field of wood preservation in order to achieve greatly increased penetration rates of preservative liquid under pressure into the wood and to avoid bleeding of the impregnated liquid from the cells. The invention is also applicable to the field of recovery of crude petroleum from wells, utilizing a water flooding technique for additional recovery of petroleum. The method and apparatus of the present invention are also applicable to wood pulping processes for accelerating the injection of delignifying chemicals into intimate contact with the wood microstructure, and generally to any process where a porous substrate is to be treated with an impregnant liquid, requiring displacement of occluded air from the porous substrate.

In typical commercial plants processing wood by impregnation, a uniform hydrostatic pressure is generally applied to the porous logs or lumber after they have been put into a closed vessel. Occluded air in the wood cells is trapped by the liquid penetrant, and the pressure of the occluded air rises until it is approximately equal to the hydrostatic pressure of the liquid treating agent, tending toward an equilibrium condition where the rate of injection of the treating medium is reduced. In order to achieve adequate hydrostatic differential pressure for penetration and retention in the treating medium, relatively long cycles are used, usually ranging from 2% to twelve hours, depending upon the type and the condition of the wood. Such treating cycles usually include slow periodic pressure increases to prevent the cells from being blocked to penetration by the liquid media, and to prevent cell collapse.

There are several instances in the prior art which suggest the supplementation of the hydrostatic pressure in the vessel with externally applied mechanical energy. One such disclosure appears in Page and Reed U.S. Pat. No. 3,467,546 which suggests the use of sawtooth shock waves applied at resonance conditions. Another such disclosure is to be found in Bodine U.S. Pat. No. 3,410,532 which deals with a mass-eccentric-rotary driven resonant apparatus operating to generate an acoustical circuit through submerged double force cones.

SUMMARY OF THE INVENTION The present invention provides an improvement on prior art methods and apparatus for impregnating wood and the like with treating liquids which significantly reduces the treating times, produces a more thoroughly impregnated product, and provides less bleeding after impregnation.

The apparatus employed in the present invention produces pressure pulses which are needlelike in configuration and which continuously vary in amplitude. In the preferred form of the present invention, the pressure pulses take the form of trains of pulses whose envelope defines a low-frequency variation with respect to time, being on the order of l to 1/1000 or so of the frequency of the individual pulses making up the train. Such pressure train pulses are quite analogous to modulated radio waves in which audiofrequency intelligence is superimposed on a high-frequency carrier wave.

I have found that pressure pulses of this nature have significant advantage when applied to liquid media under pressure for impregnating porous material, since they apparently create an excited and perturbed state in the pores of the substrate causing occluded air in the porous substrate to be expelled. The air bubbles so expelled from the substrate quickly coalesce and are continuously bled from the highest point in the pressure vessel, thus providing a substantially continuous liquid phase in the vessel, without massive air bubbles trapped at the top of the vessel.

structurally, the preferred device used in accordance with the invention includes a cylinder which is arranged to be secured to a pressure vessel in fluid communication therewith, and a free piston reciprocable in the cylinder, the piston having one end exposed to the pressure conditions in the vessel. A striker pad is arranged for reciprocation in the cylinder and has one end arranged to impact the opposite end of the free piston. A rod cooperates with the striker pad to impact the opposite end of the striker pad, the rod being slidable in a stationary sleeve, A bias spring acts on the striker pad to urge the striker pad toward the free piston, in opposition to the fluid pressure acting on the free piston. A damping spring arrests the motion of the rod and returns it toward the striker pad. An impacting means such as an air hammer periodically strikes the rod at a random or a programmed sequence. The damping spring has a fundamental frequency differing from the fundamental frequency of the piston-striker pad-bias spring assembly providing for the piston to deliver variable amplitude energy pulses into the treating vessel.

Other objects, features and advantages of this invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with'the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:

FIG. 1 is a view partly in elevation and partly in cross section of a transducer assembly which can be used in the practice of the present invention;

FIG. 2 is a cross-sectional view taken substantially along the line IIII of FIG. 1 on a slightly enlarged scale;

FIG. 3 is a view taken substantially along the line III-Ill of FIG. 1;

FIG. 4 is a plan view, on an enlarged scale, of the ballistically shaped lug employed to limit travel of the piston in the cylinder;

FIG. 5 is a fragmentary view in elevation, on an enlarged scale, illustrating the position of the lug within the cylinder; and

FIG. 6 is a schematic illustration of the elements of a pneumatic control system which can be used to actuate the air hammer employed with the transducer devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The transducer device of FIG. 1 is arranged to be welded or otherwise secured to a pressure vessel V in which logs or other forms of wood are immersed in a liquid treating agent, the entire vessel being pressurized to a value in the range from about 15 to 700 p.s.i. gauge and preferably at a value of about 60 to 250 p.s.i. gauge. The transducer assembly includes a barrel 10 which is welded or otherwise secured to the pressure vessel such that its forward lip is placed at or very close to the tangent of the inner surface of the pressure vessel. The barrel 10 has an annular flange portion 11 which abuts an annular flange portion 12 of a cylinder 13 coaxial with the barrel l0. Bolts 14 or other suitable securing means are provided to lock the two abutting flanges together.

At its rearward end as viewed in FIG. 1, the sleeve 13 is provided with an annular flange 15 which abuts against a closure plate 16 closing off the rear end of the sleeve 13. A plurality of restraining rods 17 extend from the annular flange 15 through a rear support plate 18 against which they are secured by means of nuts 19.

To further strengthen the assembly, there is provided a channel 20 having an end face 21 secured to the flange 11 as best illustrated in FIG. 2 of the drawings.

A free piston 22 with its associated piston rings 23 is arranged for reciprocation within the cylinder 13. A striker pad 24 is also mounted for reciprocation within the cylinder 13 and engages the rear end of the free piston 22. The striker pad 24 has a reduced diameter neck 25 against which a striker rod 26 is arranged to impact. The striker rod 26 is slidably received within a sleeve 27 secured to the end plate 16 and is provided with a packing gland 28 for sealing purposes. A bias spring 29 acts between the striker pad 24 and the sleeve 27 and urges the striker pad 24 and the free piston 22 in opposition to the fluid pressure on the end of the free piston 22 which communicates with the pressurized interior of the treating vessel.

The cylinder 13 is provided with a vent 30 to vent gas to the atmosphere, and also with a scavenger port 31 for disposing of any liquid treating agent which may be blown by the free piston. To permit removal of such a liquid, the striker pad is provided with a series of spaced circumferential notches 32 to deliver this liquid into the bias spring housing. The liquid recovered through the scavenger port 31 can be recirculated back into the pressure vessel.

The limit of travel of the free piston 22 within the cylinder 13 is defined by positioning of a ballistically shaped lug 33 at the forward end of the cylinder 13, as best illustrated in FIGS. 4 and 5. This lug 33 has a streamlined configuration which provides a minimum amount of frictional drag to fluid flow as the piston 22 reciprocates in the cylinder. Spacer lugs 34 (FIG. 3) are used to position the cylinder 13 concentrically withthe barrel 10.

The striker rod 26 has a collar 35 formed thereon and a reduced diameter stub shaft 36 which is tightly received within a socket 37 of an air hammer assembly generally indicated at numeral 38 in the drawings. A dampening spring 39 extends between the collar 35 and a washer 40 which is freely moved along that portion of the striker rod 26.

For the purposes of the present invention, it is important that the dampening spring 39 have a fundamental frequency which differs from the fundamental frequency of the free piston'striker pad-bias spring assembly. Upon impacting by the air hammer 38, the pulses which result vary in amplitude to produce what amounts to a modulated train of sharply peaked pulses which have been found to substantially improve the impregnation capabilities of the transducer system.

Actuation of the air hammer 38 can also be accomplished at variable rates through the type of pneumatic control system shown in FIG. 6 of the drawings. The primary air supply enters through a conduit 41 with a portion thereof passing through a driven rotary valve 42 which can be driven at a variable rate of speed. A portion of the inlet air is bypassed by means of a conduit 43 through a bypass valve 44. The pulses of air passing through the rotary valve 42 are under control of a throttling valve 45 which delivers the air to a conduit 46 feeding the air hammer 38.

In operation, with the transducer assembly rigidly secured to the wall of the pressure vessel so that the forward end of the free piston 22 is in open fluid communication with the contents of the vessel, the liquid pressure within the vessel moves the piston 22, the striker pad 24 and the rod 26 to the right, thereby compressing the bias spring 29 until the force from the spring equals the force applied by the fluid pressure. Then, when a blow is struck by the hammer 38, the piston 22 is rapidly accelerated to a high velocity. The movement of the free piston 22 pushes ahead of it a sharp, needlelike pressure pulse. When the force of the hammer blow has been expended, and the force available from the bias spring 29 has been relieved, the free piston is snubbed to a halt. The volume of liquid media displaced by the movement of the free piston 22 is largely injected into the porous substrate, but since the pump feeding the pressure vessel runs continuously, that volume of liquid is constantly being replenished. in the return stroke, when the pressure inside the vessel again forces the free piston 22 to the right as shown in FIG. 1, the piston is accelerated at a substantial velocity (augmented by energy from the pressure pump) which may be equal to or exceed the ini tial velocity from the hammer blow. As the free piston 22 moves in the opposite direction, it forms a sharp-sided negative pressure pulse which may be about to 500 lbs. p.s.i. gauge below the hydrostatic working pressure in the vessel.

The rate of actuation of the air hammer 48 is maintained higher than the fundamental cyclic rate of the pulse generator. Thus, as the free piston 22 starts its rebound and before it reaches the position of maximum displacement in the cylinder, another hammer blow is struck. This nonresonant type of actuation provides pressure pulse heights of varying instantaneous peak pressure.

The rebound of the free piston 22 accelerates the striker rod 26 to a high velocity. As the free piston 22, and the striker pad 24 reach the bias spring 29 and are thereby slowed, the striker rod 26 separates from this mechanical train, continuing it rearward movement. The dampening spring 39 serves to arrest the movement of the striker rod 26. Since the fundamental resonance period of the dampening spring 39 differs from the fundamental frequency of the free piston 22striker pad 24 bias spring 29 assembly, the position of the striker rod 26 is caused to vary as the sum or difference of the two fundamental resonance frequencies.

The actual conditions of operation will vary, of course, depending upon the porosity of the substrate and other factors. Generally, however, it is advisable to treat wood with pulses which have peaks such that they vary the pressure in the vessel from 5 to 500 pounds p.s.i. gauge above and below the static pressure existing in the vessel. The best results are also obtained when the pressure pulses from the transducer deliver an average force of from 0.00006 to 0.0006 foot pounds per pulse per square inch of surface area of substrate being treated. The pulse repetition frequency can also be varied significantly, as in a range from about 0.01 to 10,000 pulses per second.

The cells in the microstructure of wood are connected through openings called pits. Each pit has a border or has paired borders, depending upon the species of wood. In each pit there is a pit membrane which has a near solid, thickened wafer in its center, called a torus which is held in place by a circular network of very fine elastic filaments called margo." The pit membranes in wood under constant hydrostatic pressure act in the nature of check valves, sharply attenuating the passage of the liquid penetrating medium. Under modulated pressure pulse excitation of the type described herein, the pits are apparently alternatively slammed open and shut, permitting liquid media of flow into the cells. Photomicrographs of cells in wood treated by such pressure pulses show them to be in an unaspirated or open condition.

The pit channels are quite small, measuring from about 0.010 to 3.5 microns depending upon the species of wood. When liquid is forced with a static pressure through such openings, an air-liquid meniscus forms on the downstream side which with its surface tension exerts a back pressure, blocking the flow of liquid media.

The modulated pressure pulse train used in this system has peak pressures sufiicient to accelerate a column of liquid medium through the minute openings, overcoming the surface tension force developed by the air-liquid menisci. The liquid thus driven through the pit openings is converted into fine droplets and forms a liquid-in-air suspension which is quite unstable inside the cell lumen. This quickly changes into an airin-liquid emulsion which is more stable. Variable pneumatic rebounds from the force of the modulated pressure pulse train cause part of the air-in-liquid emulsion to be forced back into the cell from which it came, carrying occluded air with it, out of the center section of the wood under treatment.

Successive variable intensity impacts of the modulated pressure pulses replenish the various excited" states of the liquidin-air suspension, and the air-in-liquid emulsion. Without the train of modulated pressure pulses, the system soon returns to its original state in which the air and liquid stratify into separate phases. In this state, the forces from surface tension at the air-liquid menisci develop again. This accounts for the presence of high occluded hydrostatic pressure in the center of wood pieces which are conventionally treated.

Where the wood is treated with modulated pressure pulses of the type described herein, and the excitation is maintained as the pressure is reduced in the vessel, no comparable occluded pressure is trapped in the center of the wood.

Round wood pieces such as poles or piling treated with oiltype liquid preservatives under this type of modulated pressure pulse excitation on exposure do not show a tendency to bleed or exude part of the injected oil. Such pieces when conventionally treated have a tendency often to display bleeding in the form of an oily exudate on the surface.

Proof of the achievement of adequate excitation may be obtained by plotting the uninjected portion of the gross required liquid medium, expressed as percent of the gross required, against the sum of time increments of the pressure cycle on semilogarithm graph paper. When the modulated pressure pulse excitation in the wood pieces is adequate, a near straight line plot results. The slope of the plotted line is related to the viscosity of the air-in-liquid emulsion and of the summation of the cross-sectional areas and lengths of all pores available for penetration into the porous substrate If the plot is a straight line, this indicates that blockage of injection from the check valve action of pit membranes and from surface tension generated by air-liquid minisci is not significant and that the level of excitation in the wood pores as a result of the application of the modulated train of pressure pulses is adequate for the charge under treatment.

l claim as my invention:

1. The method of impregnating a porous substrate with a treating liquid which comprises immersing said substrate in a body of treating liquid in an enclosed treating vessel, pressurizing the liquid in said vessel to a value in the range from 15 to 700 psi. gauge, and subjecting the contents of said vessel to repeated pressure pulses of varying amplitude in consecutive cycles to increase the rate of impregnation of treating liquid into said substrate.

2. The method of claim 1 in which said pulses occur at a rate offrom 0.0l to 10,000 per second.

3. The method of claim 1 in which said pulses have peaks which vary the pressure in said vessel from 5 to 500 pounds p.s.i. gauge above and below the static pressure existing in said vessel.

4. The method of claim 1 in which said pressure pulses deliver an average force of from about 0.00006 to 0.0006 foot pounds per pulse per square inch of surface area of substrate being treated in said vessel.

5 The method of claim 1 in which said pulses have cyclically increasing and decreasing peaks, thereby forming a wave train corresponding to a series of alternately positive and negative pulses having a relatively low-frequency modulation envelope.

6. The method of claim 1 in which said porous substrate consists of wood. 

2. The method of claim 1 in which said pulses occur at a rate of from 0.01 to 10,000 per second.
 3. The metHod of claim 1 in which said pulses have peaks which vary the pressure in said vessel from 5 to 500 pounds p.s.i. gauge above and below the static pressure existing in said vessel.
 4. The method of claim 1 in which said pressure pulses deliver an average force of from about 0.00006 to 0.0006 foot pounds per pulse per square inch of surface area of substrate being treated in said vessel.
 5. The method of claim 1 in which said pulses have cyclically increasing and decreasing peaks, thereby forming a wave train corresponding to a series of alternately positive and negative pulses having a relatively low-frequency modulation envelope.
 6. The method of claim 1 in which said porous substrate consists of wood. 